1. Field of the Invention
The present invention relates to tone correction processes that correct the tone of images formed by an image forming apparatus.
2. Description of the Related Art
Electrophotographic image forming apparatuses include developing units that hold two-component developer containing a nonmagnetic toner and a magnetic carrier. Such an image forming apparatus forms images using the toner held in the developing unit. By controlling a ratio of toner mass to the total mass of the developing material held in the developing unit (called a “T/D ratio” hereinafter) to achieve a target ratio, the image forming apparatus controls the density of the images it forms to achieve a target density. This control is referred to as Automatic Toner Replenishment (ATR) control.
Japanese Patent Laid-Open No. 2004-271834 discloses an image forming apparatus that controls an amount of toner with which a developing unit is replenished based on a result of measuring a measurement image in order to control the T/D ratio of developing material held in the developing unit to achieve a target ratio.
The image forming apparatus expresses the tone of images using an area coverage modulation method. For example, the image forming apparatus detects a measurement image formed using predetermined process conditions and then executes tone correction control that generates conversion conditions for converting image data based on a result of the detection.
Although the T/D ratio is adjusted through the ATR control, toner scatter will occur with ease if the T/D ratio exceeds an upper limit value. This can result in the interior of the image forming apparatus being soiled by toner, images becoming fogged and soiled, and so on. On the other hand, carrier adhesion will occur if the T/D ratio drops below a lower limit value. Carrier adhesion is a phenomenon in which the carrier is developed along with the toner. If carrier adhesion occurs, image problems in which the image appears patchy can occur as a result. Accordingly, the T/D ratio is adjusted so as not to exceed or drop below the upper limit value and the lower limit value, respectively. The upper limit value and the lower limit value will be referred to as “T/D ratio limiters”.
However, in the case where the T/D ratio is restricted to the upper limit value, it may become impossible to suppress the toner from taking on excessively high charge. On the other hand, in the case where the T/D ratio is restricted to the lower limit value, it may become impossible to increase the toner charge amount to a target amount. In this manner, ATR control cannot correct the T/D ratio beyond the T/D ratio limiters, making it impossible to control the toner charge amount to the target amount; as a result a difference between a density of an output image and a target density may increase.